Because of the high complexity of modern integrated circuits, and the delicacy of the processes by which they are formed, they are traditionally inspected at many different times during fabrication. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
As used in the art, the term “inspection” is typically limited to optical inspection of the integrated circuits, rather than an electrical “inspection,” which is typically referred to as “testing.” Inspection is also performed on other types of items that are used in the integrated circuit fabrication process, such as masks and reticles. As used herein, the term “substrate” applies without limitation to integrated circuits, the wafers on which they are formed, masks, and reticles.
Because feature size continues to drop, optical inspection becomes increasingly harder to perform. One challenge is to provide a sufficient amount of light at a wavelength that is necessary to resolve the small features to be inspected. If there is an insufficient amount of light provided, then sensor noise tends to limit the performance of the inspection system. This is generally referred to as having an excessively low signal to noise ratio, where the optical signal that is produced is too low in comparison to the optical and electrical noise inherent in the inspection system.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.